Field of the Invention
The present invention relates to golf balls. Specifically, the present invention relates to improvement of aerodynamic characteristic of golf balls.
Description of the Related Art
Golf balls have a large number of dimples on the surfaces thereof. The dimples disturb the air flow around the golf ball during flight to cause turbulent flow separation. This phenomenon is referred to as “turbulization”. Due to the turbulization, separation points of the air from the golf ball shift backwards leading to a reduction of drag. The turbulization promotes the displacement between the separation point on the upper side and the separation point on the lower side of the golf ball, which results from the backspin, thereby enhancing the lift force that acts upon the golf ball. Excellent dimples efficiently disturb the air flow. The excellent dimples produce a long flight distance.
A polyhedron is used for arrangement of dimples. The polyhedron is inscribed in a phantom sphere of a golf ball. A large number of sides of the polyhedron are projected on the surface of the phantom sphere by a light beam travelling from the center of the phantom sphere in a radius direction. A large number of comparting lines are obtained on the surface of the phantom sphere by the projection. By the comparting lines, the surface of the phantom sphere is divided into a large number of units (spherical polygons). A large number of dimples are arranged in one unit to obtain a dimple pattern. The dimple pattern is developed over the other units to obtain a dimple patter of the whole golf ball. This dimple patter is referred to as a polyhedron pattern.
A dimple pattern referred to as a hemispherically divided pattern is adopted in commercially available golf balls. In designing the pattern, a hemisphere (half of a phantom sphere) is divided into a plurality of units by a plurality of longitude lines. Each unit has a shape of a spherical isosceles triangle. A large number of dimples are arranged in one unit to obtain a dimple pattern. The dimple pattern is developed over the other units. The development is obtained by rotating one unit pattern about a line passing through a north pole and a south pole. By the rotation, a dimple pattern of the whole golf ball is obtained. The pattern of the golf ball is rotationally symmetrical.
The polyhedron pattern is monotonous. In the polyhedron pattern, the turbulization is insufficient. The hemispherically divided pattern is also monotonous. In the hemispherically divided pattern, the turbulization is insufficient.
There have been various proposals for improvement of the hemispherically divided pattern. US2007/0149321 (JP2007-175267) discloses a dimple pattern in which the number of units present in a high-latitude region is different from the number of units present in a low-latitude region. US2007/0173354 (JP2007-195591) discloses a dimple pattern in which the number of types of dimples present in a low-latitude region is greater than the number of types of dimples present in a high-latitude region. US2013/0196791 (JP2013-153966) discloses a dimple pattern in which the density of dimples is high and variations in sizes of dimples are small.
US2009/0191982 (JP2009-172192) discloses a golf ball that has randomly arranged dimples. The dimple pattern of the golf ball is referred to as a random pattern. The random pattern is not monotonous. US2012/0004053 (JP2012-10822) also discloses a golf ball having a random pattern.
Golf players place importance on flight distance in a shot with an iron club as well as flight distance in a shot with a driver. Players particularly place importance on flight distance in a shot with a middle iron and a long iron. A spin rate of a golf ball in hitting with a middle iron is high. If a golf ball having above mentioned improved hemispherically divided pattern is hit with a middle iron, an excessive lift force is generated. The lift force may cause rising of the golf ball during flight. The rising impairs flight distance performance. In addition, in the golf ball, the flight distance depends largely on the rotation axis of backspin. In other words, the golf ball is inferior in stability of flight distance.
As already mentioned, the random pattern is not monotonous. However, the density of dimples in the random pattern is low. In the pattern, suppression of drag is insufficient. When the golf ball is hit with a middle iron, great flight distance cannot be achieved.
An objective of the present invention is to provide a golf ball that is excellent in flight distance performance and flight distance stability in a shot with a middle iron.